Magneto-optic disks have been noted as a medium which is rewritable, has large recording capacity and is highly reliable, and they have been put to use as memories for computers and the like. However, as the amount of information is increased and devices are miniaturized, a recording/reproduction technology with higher density is increasingly demanded.
High-density recording/reproduction technology is constituted by device techniques and medium techniques. The former techniques include optical super-resolution for obtaining a condensation spot exceeding the diffraction limit of laser light, reduction in laser beam wavelength, and the like. The latter techniques include reduction of medium pitch, improvement of reproduction resolution by means of magnetic multilayered film, and the like. The technique for improvement of reproduction resolution by means of magnetic multilayered film employs the fact that the temperature within a laser spot is maximized around its center or presents a Gaussian distribution to selectively transfer the state of a recording layer to a reproduction layer and read the state of the reproduction layer, and at present there are three main techniques, i.e. FAD, RAD and CAD (Center Aperture Detection). According to these techniques, the front or rear side or the center of a laser spot can be masked to provide a substantial reproduction area that is smaller than the diameter of the laser spot. This results in an improved reproduction density.
Conventionally, a magneto-optic disk employed in optical super-resolution includes a substrate, a base layer formed on the substrate, a reproduction layer formed on the base layer, a recording layer formed on the reproduction layer, and a protection layer formed on the recording layer. A magneto-optic disk with such a structure typically has a recording capacity of approximately 5 Gbytes.
The FAD, RAD and CAD systems described above cannot provide a magneto-optic disk with sufficiently large recording capacity, since the laser beams radiated for recording cause enlargement of the recording area that exceeds the Curie temperature. The laser beams radiated for reproduction also result in enlargement of the reproduction area exceeding the Curie temperature and hence low carrier-noise (CN) ratio in reproduction.